This invention is in the field of oil and gas prospecting, and is more specifically directed to the interpretation of well log signals to detect potential hydrocarbon reservoirs.
In the field of oil and gas prospecting and production, geophysical well logging is a commonly-used method of obtaining information about subsurface geology in the vicinity of a wellbore (or borehole), during and after the drilling process. As is fundamental in this field, well logging is accomplished by the lowering of a logging tool into the wellbore, and by operating the logging tool to obtain measurements at selected depths therewithin, for example as the tool is being withdrawn from the wellbore.
Many technologies are commonly used in obtaining various types of well log measurements. These technologies range from a simple caliper that directly measures the diameter of the wellbore over its depth, to the commonly used gamma-ray log that measures the natural radioactivity of the formations surrounding the wellbore, from which certain important formation types such as shales may be identified. Other types of well log measurements include density logs derived from gamma-ray backscatter measurements, sonic logs from which acoustic velocity may be derived as a function of depth, and the like. The resulting well log measurements are typically "tied" to seismic surveys at the drilling locations, thus calibrating the seismic surveys to the more direct physical measurements provided by conventional well logs.
As is also well known in the art, fractures in the earth are of particular interest in the prospecting of oil and gas reserves. Fractures are of particular significance in regions of the earth at which the subsurface formations otherwise have low permeability and low porosity, as the ability to retrieve oil and gas products from such formations depends directly upon the number and size of fractures in those formations. Secondary recovery, in which fluids are injected into producing formations to displace and thus retrieve hydrocarbons after reservoir pressure has dropped below the producing threshold, also greatly depends upon the presence of fractures. Information such as the depth at which fractures are intersected by the wellbore, as well as attributes of the fractures (e.g., strike and dip), are useful in analysis of the formation.
Fracture analysis has historically depended, to a large degree, upon core samples that are directly obtained from the formations surrounding a wellbore. However, core sampling is an expensive process, and one that is inherently limited to small sample locations. Furthermore, heavily fractured formations are often quite fragile, so that the obtaining of core samples therefrom can be difficult. In addition, considering that the core sample, once removed, is no longer subject to the stresses that were present in the earth itself, any sort of pressure or stress analysis is rendered suspect.
Well logs based upon the imparting of Stoneley waves (or "tube" waves) along a wellbore have been used in the art to locate fractures intersected by the wellbore. For example, the well log tool described in U.S. Pat. No. 4,831,600 includes a source for generating Stoneley waves, and an array of receivers spaced along the tool. This tool is used to sense the reflection of imparted Stoneley waves from fractures intersected by the wellbore. The reflection of Stoneley waves from such fractures is caused by fluid movement into the fracture, which generates a secondary Stoneley wave that is proportional to the width of the fracture. As described therein, analysis of the reflected signals allows the analyst to locate the depth of the fractures, as well as the width of the fracture (derived from an estimated reflectivity parameter). Further description of this conventional approach is provided in Hornby, et al., "Fracture evaluation using reflected Stoneley-wave arrivals", Geophysics, Vol. 54, No. 10 (October 1989), pp. 1274-1288.
It has been found, however, that the Stoneley wave measurement and analysis described in U.S. Pat. No. 4,831,600 and in the Hornby, et al. article is unable to distinguish between a true formation fracture, on one hand, and a cave, washout, breakout, or other artifact of the drilling process, on the other hand. It has been observed, in connection with the present invention, that any change in wellbore diameter, not just an intersected fracture, can cause the generation of a secondary Stoneley wave. By way of definition, a cave is a local enhancement of the borehole that is caused by the drilling process. Various types of caves are commonly encountered in the drilling of a borehole. One type of cave, referred to as a washout, is a local widening of the wellbore diameter due to an overpressure in the drilling fluid relative to the surrounding formation; a breakout corresponds to a shallow caving along the wellbore due to horizontal stress anisotropy as a result of the drilling operation. A cave may also be a drilling-induced enhancement of the opening of a formation fracture. Conventional Stoneley wave logging of wellbores having these drilling artifacts may indicate a formation fracture at locations of these artifacts (i.e., false positives), or may otherwise overestimate the fracture density along the wellbore.
It is therefore desirable to be able to distinguish between true formation fractures and drilling artifacts in Stoneley wave well logging of a wellbore.
It has further been discovered, in connection with the present invention, that Stoneley waves can also be reflected from locations along a wellbore at which a sharp change in lithology is present. Such reflections are due to discontinuities in the Stoneley wave velocity through the surrounding formation. While knowledge of the locations of lithology changes along a wellbore is of some importance, this information can be acquired by other techniques (such as velocity logs), and as such it is therefore desirable that reflections from lithology changes not interfere with reflections from fracture formations in the interpretation of the well log.
By way of further background, differences in the frequency response to Stoneley waves between true fractures and drilling artifacts such as caves and washouts have been discovered. Attention in this regard is directed to Kostek, et al., "The interaction of tube waves with wellbore fractures, Part I: Numerical models", Geophysics, Vol. 63, No. 3 (May-June 1998), pp. 800-808, and to Kostek, et al., "The interaction of tube waves with wellbore fractures, Part II: Analytical models", Geophysics, Vol. 63, No. 3 (May-June 1998), pp. 809-815, in this regard. As described therein, a washout not at a fracture location has poor low frequency response and good high frequency response, while a fracture has good low frequency response and poor high frequency response; a washout at a fracture location has the good low frequency response of a fracture and the good high frequency response of a washout. These references disclose that behavior of the reflection coefficient versus frequency can be used to identify regions where only washouts or caves exist. In this regard, attention is directed to the Kostek, et al. article at page 815.
By way of further background, the compensation of Stoneley wave well log data for the effects of borehole enlargements using forward modeling of the Stoneley wave based on the caliper log, is known in the art. Examples of this approach are disclosed in Tezuka, et al., "Modeling of low-frequency Stoneley-wave propagation in an irregular borehole", Geophysics, Vol. 62, No. 04 (1997), pp. 1047-1058, and in Endo et al., "Fracture evaluation from inversion of Stoneley transmission and reflections", 48th SPE Annual International Meeting, (Soc. Petr. Eng., 1998). According to these references, the field data are compensated for borehole enlargements using synthetic Stoneley wave responses based on a caliper log, and then subtracting that synthetic response from the field data. The corrected data are then processed for Stoneley wave reflectivity over a single frequency band. These conventional procedures depend upon very accurate measurements of the borehole diameter with the caliper log, and also require full-fidelity matching of the synthetic data with field data. However, a caliper log may not accurately and entirely represent the whole borehole; for example, caves that are caused by local enlargement of the fracture will not normally be picked up by caliper logs, but will still affect the Stoneley wave response.